Fluorescence endoscopy utilizes differences in the fluorescence response of normal tissue and tissue suspicious for early cancer as a tool in the detection and localization of such cancer. The fluorescing compounds or fluorophores that are excited during fluorescence endoscopy may be exogenously applied photoactive drugs that accumulate preferentially in suspicious tissues, or they may be the endogenous fluorophores that are present in all tissue. In the latter case, the fluorescence from the tissue is typically referred to as autofluorescence or native fluorescence. Tissue autofluorescence is typically due to fluorophores with absorption bands in the UV and blue portions of the visible spectrum and emission bands in the green to red portions of the visible spectrum. In tissue suspicious for early cancer, the green portion of the autofluorescence spectrum is significantly suppressed. Fluorescence endoscopy that is based on tissue autofluorescence utilizes this spectral difference to distinguish normal from suspicious tissue.
Since the concentration and/or quantum efficiency of the endogenous fluorophores in tissue is relatively low, the fluorescence emitted by these fluorophores is not typically visible to the naked eye. Fluorescence endoscopy is consequently performed by employing low light image sensors to acquire images of the fluorescing tissue through the endoscope. The images acquired by these sensors are most often encoded as video signals and displayed on a color video monitor. Representative fluorescence endoscopy video systems that image tissue autofluorescence are disclosed in U.S. Pat. No. 5,507,287, issued to Palcic et al.; U.S. Pat. No. 5,590,660, issued to MacAulay et al.; U.S. Pat. No. 5,827,190, issued to Palcic et al.; and U.S. Pat. No. 5,647,368, issued to Zeng et al. Each of these patents is assigned to Xillix Technologies Corp. of Richmond, British Columbia, Canada, the assignee of the present application. While the systems disclosed in the above-referenced patents are significant advances in the field of early cancer detection, improvements can be made.
These aforementioned systems are typically used in conjunction with an endoscope to which a camera containing low light sensors is attached or utilize a video endoscope with the camera located at the insertion end of the endoscope. In particular, it is desirable to reduce the size, cost, and weight of the camera described for these systems. Since fluorescence endoscopy is commonly performed as an adjunct to conventional white light endoscopy, it is also desirable for the system to be capable of acquiring both color and fluorescence images with the same camera and light source. It is also desirable to optimize such a fluorescence endoscopy video system to detect various types of cancer in different organs and to provide features so that it is easily calibrated for use with different types of endoscopes. It is also desirable that such a system be compatible for use with exogenously applied photoactive drugs. Finally, there is a need for a system in which the contrast between normal and suspicious tissue may be enhanced in the displayed fluorescence images.